Amplification system



Feb. 10,1953

1. E. GROSDOFF AMPLIFICATION SYSTEM Filed May 24, 1951 40 5 J1 .Pi/W/YG 21/15! I J'fll/ECE 7 I NVENTO R 620100;;

ATTORNEY Patented Feb. 1%, 1953 AMIPLIFICATION SYSTEM Igor E. Grosdoif, Princeton, N. .L, assignor to Radio Corporation of America, a corporation of Delaware Application May 24, 1951, Serial No. 221,990

6 Claims.

This invention relates to amplification systems and more particularly to an improvement in amplification systems used for driving capacitive type loads.

In applying high frequency pulse type signals to a capacitive type of load such as the input to a number of tubes, a long transmission line, etc., it is desirable to drive the load with the best possible waveshapes. In order to achieve this, the load may be charged through the electron discharge path of one tube and may be discharged through the electron discharge path of a second tube. The charge and discharge tubes are usual- 1y connected in series and the capacitive load is connected across one of them. In this type of operation the discharge tube is usually driven by one driver tube and the charge tube is usually driven by another driver tube. However, it has been found that, with this type of circuit and with the type of tube required for charging and discharging a large capacitive load, unless a very large driving signal is used, it is difiicult to completely cut off the discharge tube during the charging of the load. Without such complete cutofi, a large unnecessary current is always flowing. This current serves to spoil the waveshape of the charging pulse signal. To obtain complete cutoff, large and expensive driving tubes are required.

It is an object of the present invention to provide an improved amplifier system for driving a load which draws substantially no unnecesary current when being driven.

It is a further object of the present invention to provide an improved amplifier system for driv-v :zs

ing a load which leaves the driving waveshape substantially unafiected.

t is still a further object of the present invention to provide a novel and inexpensive amplifier system for driving a capacitive load which requires fewer components than heretofore.

These and further objects of the present invention are achieved by connecting a first and a second tube in series. A capacitive-load may be connected across either the first or the second tube. The control grid of the first tube is connected to the screen grid of the second tube. Potentials are applied to both tubes to hold the second tube non-conductive and the first tube conductive. A resistor is connected between the first tube cathode and the second tube screen grid and driving signals are applied to the second tube control grid. The value of the resistor is selected so that the second tube screen current, which flows through the resistor when the secand tube is rendered conductive, establishes a bias voltage which is sufficient to bias ofi the first tube until the second tube ceases conduction. The load discharges through the tube across which it is connected when that tube is rendered conductive and, charges through the other tube when the other tube is rendered conductive.

The novel features of the invention, as well as the invention itself, both as to its organization and method of operation, will best be understood from the following description, when read in connection with the accompanying drawing, in which there is shown a circuit diagram of an embodiment of the invention.

A first tube II] has its anode 12 connected to a source of' operating potential or 13+, its suppressor grid I4 connected directly to its cathode 20, and its screen grid 15 connected to a source of operating potential or B+ through a voltage dropping resistor 22. The first tube control grid l8 and cathode are connected to a second tube in a manner to be hereinafter described.

A capacitive type of load, represented by a capacitor 24 in a dotted rectangle, is connected across the first tube [0 between its cathode 2B and its anode [2. A second electron discharge tube has its anode 32 connected to the cathode 20 of the first tube-Hi. Also, a high value re sistance 42 is connected from the second tube anode 32 to the source of 3+. The second tube 30 has its suppressor grid 34 connected to its cathode 40, and its screen grid 35 is connected to the control grid l8 of the first tube Ill. Its screen grid 36 is also connected through a low value resistance 44 and a peaking coil 45 to the cathode 20 of the first tube. The cathode 48 of the second tube is connected to ground. The control grid 38 of the second tube is connected to a source of high frequency driving pulses 48 as well 2 as to a source of negative bias which normally maintains the tube non-conductive.

The value of the resistor 44 connected to the screen grid 36 of the second tube 38 is chosen as low as possible with the sole limitation that when the screen grid of the tube draws current, this current, being drawn through the resistance, should establish a bias potential sufiicient to cut oil the first tube I9. The peaking coil 45 is provided to shape the leading edge of the pulse of current drawn by the screen grid of the tube to be more steep when the second tube starts conducting and thereby the first tube is out off more quickly.

The high value resistor 42 has its resistance value selected so that when the capacitive load is charged up to its plateau voltage, this plateau voltage is not permitted to decrease to any great extent. Stated another way, the value of the high value resistance is so selected that its time constant, when taken together with the load capacitance, permits very little voltage decrease across the load capacitance during the period 01' the signal plateau interval. Another limitation on the value of the resistance 42 is that it must not be excessively high, since during this plateau interval, the second tube is permitted to draw a minimum of plate current. This resistor may be omitted, but it is desirable to maintain the plate at some potential at all times for reference level purposes, and therefore the resistor 42 is provided.

In operation, a positive pulse is applied from the high frequency driving pulse source 48 to the control grid of the second tube 3-9. This causes the second tube to conduct during the pulse interval. Upon conduction, the second tube screen 36 draws current through the peaking coil 45 and low value resistor 44-, thus completely biasing ofi the first tube 0. The capacitive load charges up through the second tube. Upon the termination of the driving pulse the second tube ceases conduction. Thereupon the bias, holding the first tube cutoff, is terminated and the first tube is made conductive, thus rapidly discharging the capacitive load. It is interesting to note that, because of the plate and screen characteristics of a pentode or tetrode tube, as the potential on the plate of the tube drops with the charging of the load, a, point is reached where the screen current increases much more rapidly than the plate current decreases. The result is that the screen potential goes down rapidly, pulling the first tube grid potential down with it, thus further insuring that the first tube is maintained cut off while the driving pulse is applied to the second tube control grid.

While the embodiment of the invention has been shown and described thus far with the capacitive load connected across the first tube, the system operates equally well'if'the capactive load is connected across the second tube instead. No other circuit change is required. However, in operation, the capacitive load is charged up through the normally conducting first tube. When a driving pulse is applied to the second tube control grid it goes conductive, thus cutting off the first tube and discharging the capacitive load.

For the purpose of illustrating an operative em bodiment of this invention, but not to be con- I strued as a limitation thereon, component values for an embodiment of the invention which was built are supplied. Two 6AU5 tubes were used for the first and second tubes. The 3+ supply was 300 volts and the negative bias applied to the control grid of the second tube was -l50 volts. The capacitive load was 1500 mmfd; the high value resistor was 22,000 ohms and the low value resistor was 200 ohms. The driving pulse source frequency was approximately 200 kc. The waveshape across the capacitive load showed pulses having essentially straight leading and trailing edges and the deviation of the plateau from the horizontal was less than one percent.

While the subject embodiment of the invention has been described as being best utilizable for driving a capacitive load, this is not to be taken as a limitation on the system. The present system may be used to drive any type of load or it may be used in any system where it is desired to very rapidly switch between a high impedance path in series with a load and a low impedance path in shunt with the load to a condition where the series path is low impedance and the shunt path is high impedance.

There has been described a novel, inexpensive and improved amplifier system for driving a capacitive load which draws no unnecessary current and which does not afiect the driving signal waveshape. It is to be further noted that only one of the tubes in the amplification system requires a driving tube. The other tube in the system is keyed off and on responsive to the conductive condition of the tube being driven.

What is claimed is:

1. An amplifier system for driving a load comprising a first and a second electron discharge tube each having at least a cathode, anode, control grid, and screen grid, said first tube cathode being connected directly to said second tube anode, said first tube control grid being connected directly to said second tube screen grid, said load being connected across one of said tubes, means to apply operating potentials to said first and second tubes to maintain said first tube conductive and said second tube non-conductive, means to apply driving signals to said second tube to render said second tube conductive, and means connected between said first and second tube to render said first tube non-conductive when said second tube is rendered conductive.

2. A system for driving a capacitive load comprising a first tube having anode, cathode and control grid electrodes, a second tube having anode, cathode, control grid and screen grid electrodes, said first tube cathode being connected to said second tube anode, said capacitive load being connected across one of said tubes, means to apply potentials to said first and second tubes to maintain said first tube conductive and said second tube n0nconductive, means to apply a driving signal to said second tube to render said second tube conductive, means connecting said second tube screen grid to said first tube cathode to generate a cut-off bias for said first tube responsive to current being drawn by said screen grid, and means to apply said cutoff bias to said first tube control grid.

3. A system for driving a capacitive load comprising a first electron discharge tube having, an anode, cathode and control grid, at second electron discharge tube having an anode, a cathode, a control grid and a screen grid, said capacitive load being connected between said first tube anode and cathode, said first tube cathode being connected to said second tube anode, said first tube control grid being connected to said second tube screen grid, means to apply potentials to said first and second tubes to maintain said first tube conductive and said second tube non-conductive, means to apply a driving signal to said second tube control grid to render said second tube conductive, and means coupling said first tube cathode to said second tube screen grid to apply a cutoff bias to said first tube when said second tube is rendered conductive.

4. A system as recited in claim 3 wherein said means to apply a cutofi bias to said first tube when said second tube is rendered conductive includes a resistor and peaking coil in series, said resistor having its value selected to provide a cutofi bias when said second tube screen grid draws current through said resistor.

5. A system for driving a capacitive type load comprising a first electron discharge tube having an anode, a cathode and a grid electrode, a, second electron discharge tube having an anode, a cathode, a control grid and a screen grid, said load being connected between the anode and cathode of said first tube, a first resistor connected in parallel with said capacitive load, said second tube anode being connected to said first tube cathode, a second resistor connected between said first tube cathode and said; second tube screen grid, said first tube control grid being connected to said second tube screen grid, means to apply potentials to said first and second tubes to maintain said first tube conductive and said second tube non-conductive, and means to apply a driving signal to said second tube control grid to render said second tube conductive, whereby said first tube is rendered non-conductive when said second tube screen grid draws current.

6. A system as recited in claim 5 wherein the value of said first resistor is selected to provide a time constant when considered with the capacitive load such that there is substantially no 6 change in the voltage to which said capacitor is charged during the driving signal plateau time and to permit a minimal current to be drawn by said second tube, and said second resistor value is selected to provide a cutofi bias for said first tube when said second tube screen grid current is drawn through it.

IGOR E. GROSDOFF.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 2,423,931 Etter July 15, 1947 2,489,312 Pacini Nov. 29, 1949 FOREIGN PATENTS Number Country Date 581,910 Great Britain Oct. 29, 1946 

